1. Field of the Invention
The present invention relates to unstretched and stretched cellulose acylate films produced according to a melt casting process, which are usable as optical films. The invention also relates to a method for producing the films.
2. Description of the Related Art
Cellulose acylate films heretofore used for substrates for liquid-crystal image display devices are produced by dissolving a cellulose acylate in a chlorine-containing organic solvent such as dichloromethane to prepare a dope, then casting the dope through a casting die onto a casting support, and drying it to form a film thereon. Dichloromethane has a low boiling point (about 40° C.) and its advantage is that it readily evaporates. Accordingly, heretofore, dichloromethane is favorably used as a chlorine-containing organic solvent for cellulose acylate films.
Anyone is obliged to handle the low-boiling-point, chlorine-containing organic solvent such as dichloromethane in a closed system, from the viewpoint of current environmental protection. For example, a method is employed which comprises preventing the leakage of such a chlorine-containing organic solvent in a perfect closed system, and, even if leaked out, the chlorine-containing organic solvent is adsorbed by an adsorbent in a gas-absorption column and treated therein before it goes out into external air. Further, before discharged out into external air, the chlorine-containing organic solvent is fired or decomposed with electron beams, and then the thus-decomposed solvent is discharged out. This is another method heretofore employed in the art. However, it is difficult to completely prevent the discharge of chlorine-containing organic solvents, and further studies are needed.
For preventing chlorine-containing organic solvents from being discharged out into external air, heretofore known is a method of producing cellulose acylate films not using a chlorine-containing organic solvent (JP-A-2000-352620, claim 7, paragraphs [0016], [0039] to [0040]. In this film-forming method, the carbon chain of the ester group of the cellulose acylate is prolonged and the melting point of the polymer is thereby lowered so that the polymer may be readily formed into films in a mode of melt casting. More concretely, cellulose propionate, cellulose butylate or the like is used in place of cellulose acetate so as to facilitate the melt-casting film formation.
However, when a polarizer is formed of the cellulose acylate film produced according to the method as above and when this is built in a liquid-crystal display device and used therein, then it has a drawback in that uneven display, especially remarkable uneven display occurs in the machine direction of the film. The display unevenness is especially remarkable when the film is built in a large-size liquid crystal display panel of 15 inches or more, and it is a serious problem for large-size displays and therefore requires further improvements. In addition, the cellulose acylate film produced according to the above-mentioned method has another problem in that its visual field may often vary depending on the ambient humidity change, and the film therefore requires further improvements.
The present invention has been made in consideration of the above-mentioned problems, and its object is to provide unstretched and stretched cellulose acylate films which, when built in liquid-crystal display devices, significantly relieve or solve the problems of display unevenness and humidity-dependent visibility change, and also to provide a method for producing the films.